1. Field of the Invention
This invention relates to an improved process for forming a mask on an uneven surface of a semiconductor wafer using multilayers of photoresist. More particularly, this invention relates to an improved process for etching a multilayer photoresist mask structure formed on an uneven surface of a semiconductor wafer which will minimize feature undercutting of the lower portions of the mask and also provide even loading of the photoresist during etching.
2. Description of the Related Art In the preparation of photoresist masks over stepped or uneven surfaces of a semiconductor wafer, it is known that focus (optically-related) problems can occur in the photolithographic transfer of the pattern from the photolithography mask to the photoresist layer when the photoresist layer is not flat. This results in the lack of control of critical dimensions of the features which define the performance of an integrated circuit.
Therefore, when forming a photoresist mask over an uneven surface on a semiconductor wafer in the prior art, e.g., a wafer having already formed thereon integrated circuit structure including raised lines or trenches or the like, it has become the practice to use a multilayer photoresist structure.
As shown in the prior art photolithography step of FIG. 1, a planarizing photoresist layer 10 is first formed (spun) over an integrated circuit structure 4 comprising the uneven surface portion of a semiconductor wafer. An intermediate layer 20, which may comprise an oxide such as a spin on glass (SOG) or a CVD formed silicon dioxide glass, is deposited over first photoresist layer 10 and then a second imaging layer of photoresist 30 is formed over oxide layer 20 to provide a level photoresist surface onto which may be projected a light image of the desired pattern from a photolithographic mask, using standard photolithography techniques.
Opening 40 is then photolithographically formed in upper photoresist mask layer 30, as shown in FIG. 1, by optical projection of a light pattern onto photoresist layer 30, followed by development of the photoresist as is well known to those skilled in the art.
While the foregoing practice solves the photolithography problem of attempting to form a light pattern on an uneven surface of a photoresist layer, there still remains the problem of accurately transferring to lower photoresist layer 10 the pattern photolithographically formed in upper photoresist layer 30.
FIGS. 2-4 sequentially illustrate the prior art practice of forming in lower photoresist layer 10 the pattern photolithographically formed in upper photoresist layer 30.
As shown in FIG. 2, the initial step in the prior art, following the photolithography step, was to anisotropically etch the oxide layer 20 in between the upper and lower photoresist layers through opening 40 formed in upper photoresist layer 30, using an appropriate oxide etch chemistry, thereby exposing that portion of the surface of underlying lower photoresist layer 10 through opening 40.
The etch chemistry was then changed to a photoresist etch chemistry to anisotropically etch an opening through lower photoresist mask layer 10, using opening 40 formed in upper photoresist mask layer 30 and oxide layer 20 as a mask, as shown in FIGS. 3 and 4.
However, as seen in FIGS. 3 and 4, two problems were encountered in the prior art with this process. First of all, when the etch chemistry is changed to etch photoresist, the exposed portion of lower photoresist layer 10 is etched through opening 40 simultaneous with the blanket etching of the entire surface of upper photoresist layer 30. However, when upper photoresist layer 30 has been completely removed, the etch loading on the system drastically changes, since only the relatively small surface area of photoresist layer 10 exposed by opening 40 (as well as other similar openings in oxide layer 20) is now being etched. This results in problems of control of the etch rate.
Additional problems which were encountered in the prior art using the above procedures included detection of the process end (by emission spectroscopy) due to the increase in etch/rate and the signal intensity change when layers 30 clears. Also, the change in gas phase composition, when changing the etch chemistry during the process, affects the process results, e.g., the feature profile of the resulting integrated circuit structure.
Furthermore, it will also be noted in FIG. 3 that during the anisotropic blanket etching of upper photoresist layer 30, faceting of the edge of photoresist layer 30 occurs at the corners of opening 40, as shown at 36. This, in turn, results in deflection or reflection of the etch ions from these surfaces to strike the sidewalls of lower photoresist layer 10 at 14, during the anisotropic photoresist etch step, causing a bowing effect in the sidewalls of the opening 40 being etched in lower photoresist layer 10. When this occurs, a true image of the pattern photolithographically formed in upper photoresist layer 30 will not be reproduced precisely in lower photoresist layer 10, thus defeating the whole purpose of using a multi-layer photoresist mask.